perf(files): batch session attachment validation YEF-1058

[Yevanchen]

Summary

• Replace per-attachment file and session reads with one target-session authorization check and one bulk file lookup.
• Preserve requested attachment order and duplicates, with a regression test that pins the D1 statement count.

Why

• Attachment admission previously prepared two D1 statements per attachment, up to 200 statements at the 100-file session limit. The new path prepares two statements for any batch size while retaining O(n) in-memory validation.
• Closes YEF-1058

Verification

• Commands:
  • just test-file apps/api/tests/file-session-attachment-validation.test.ts (1 passed; baseline measured 6 statements for three inputs, optimized path measures 2)
  • just test-file apps/api/tests/send-agent-session-events.test.ts (8 passed)
  • just tc-package @mosoo/api
  • just test-package @mosoo/api (1,227 passed)
  • just commit-check
  • just check (format, docs, lint, and typecheck passed; the repository test phase stopped on 11 process-watchdog failures in the untouched apps/driver submodule after 1,220 passes)
• Manual steps: N/A
• Not run: Browser/E2E flows and the final Public API contract step skipped by the failed full gate; this change does not alter UI or Public API contracts.

Impact

• User/API/contract changes: None; internal session attachment admission keeps the same return order and validation rules.
• Generated files / GraphQL / DB / lockfile: None.
• Env or config changes: None.
• Risk and rollback: Low; authorization remains explicit before the bulk read, and reverting 0deec857 restores the prior lookup path.

Review

• Closest review areas: Session file authorization, invalid attachment handling, duplicate/order preservation, and the D1 statement-count assertion.
• Known trade-offs: The two D1 reads remain sequential so target-session authorization completes before file records are loaded; the bulk result is indexed in memory in O(n).